This invention relates to gravity meters, and is particularly concerned with continuously recording gravity meters.
The concept of gravity meters has been known for some considerable time. Their object is to measure small changes in the acceleration of a mass due to gravity, known as xe2x80x9cgxe2x80x9d. However most such meters have been expensive to manufacture and are unsuitable for long-term installation in the field.
Gravity meters are used inter alia by geophysicists, especially for monitoring volcanic activity and for oil exploration. They do not normally measure the absolute value of g but changes in its value. They can be installed at a particular site for long-term measurements, or can be taken from site to site, taking measurements at each.
It is an object of the present invention to provide a gravity meter which can be made at lower cost, which is reliable and accurate in operation, and which is portable.
The concept underlying the vertical gravity meter is that, from the definition of gravity, a mass of any given size will weigh more closer to the center of the earth than it does at a greater distance from the center. Therefore, if one has a mass at 0 meters elevation (sea level) and if one raises the mass by 1 meter to +1 meter above sea level, then the mass should have decreased in weight.
With some designs of gravity meter they may have a relatively low recording resolution. It is another object of the present invention to provide a gravity meter with a higher recording resolution.
This is achieved in accordance with the invention by the use of a material whose electrical characteristics change when it is distorted, and by using changes in its electrical characteristics upon distortion to provide a measurement of changes in xe2x80x9cgxe2x80x9d.
In accordance with the invention there is provided a gravity meter comprising a casing, a vacuum tube mounted in the casing in a vibration-free manner, a sensor mechanism mounted within the vacuum tube, the sensor mechanism comprising two masses of different size acting on the respective arms of a beam balance comprising a material whose shape is arranged to change in response to changes in gravity and whose shape can be restored by the application of an electrical current thereto, and detector means arranged to provide from the restoring current an output representative of changes in gravity.
In a preferred embodiment the beam balance material is a piezoelectric material.